A varactor diode exhibits capacitance changes with changes in applied DC potential across its terminals. The capacitance of the diode is a function of the junction area, the applied potential and the type of doping. Varactor diode tuners have been known and in use for a number of years and indications are that they will render electromechanical type tuners obsolete.
A varactor tuner uses varactor diodes to provide variable capacitance for adjusting the resonant frequencies of its tunable circuits, by application of appropriate magnitude DC tuning voltages. The wide frequency spectrum encompassing the VHF and UHF television signal bands have dictated use of so-called hyper abrupt junction varactor diodes which exhibit large capacitance changes for a given tuning voltage change. (This characteristic is described as the Cmax/Cmin ratio or the C-V characteristic of the diode.) Such diodes are, in turn, expensive to make and variable in characteristics. As will be seen, hyper abrupt diodes for present day varactor tuners are carefully selected to insure matched performance among the various tuner stages.
The VHF television spectrum or range embraces three distinct bands of frequencies. The first band is from 57 MHz to 69 MHz (channels 2-4), the second band is from 79 MHz to 85 MHz (channels 5 and 6) and the third band is from 177 MHz to 213 MHz (channels 7-13). Each television channel occupies 6 MHz and the above limits are measured from the center frequencies of the channels. The overall frequency ratio, that is, the ratio of the maximum to minimum channel frequency in the VHF range is about 3.7:1 ##EQU1## The UHF range is a single continuous band extending from 473 MHz to 887 MHz and has a frequency ratio of 1.9:1.
In general, for capacitance tuning of the tuned circuits, the capacity ratio Cmax/Cmin (ratio of maximum to minimum value of tuning capacity) required is related to the square of the frequency ratio. A frequency ratio of 3:1 would dictate a capacitance ratio of 3.sup.2 :1 or 9:1, without provision for any safety margin. Even with hyper abrupt junction diodes, the 3.7:1 frequency ratio in the VHF spectrum is unattainable without bandswitching.
Bandswitching involves changing the values of one of the reactive components in the tuned circuit to enable the same value opposite sign reactance component to tune the circuit to a different resonance point. In practice the VHF spectrum is divided into two frequency sections, the "Lo" band comprising the above-mentioned first and second bands (channels 2-6) and the "Hi" band comprising the third band (channels 7-13). The Lo band channel frequency ratio (57-85 MHz) is approximately 1.5 and the Hi band channel frequency ratio (177-213 MHz) is about 1.2. The capacity ratios are, therefore, about 2.25 and 1.4, respectively. The bandswitch generally shorts out a portion of the inductance in each tuned circuit for tuning the circuits to higher resonant frequencies with the same varactor diode capacitance.
Other important factors are that the minimum tuning voltage used must be large enough to preserve the signal handling capability of the tuner and the gain and bandwidth of the tuner must be maintained. This factor, referred to as the gain-bandwidth product, is of great importance and will be seen to be directly related to the tuning capacitance and the stray capacitance present in the tuner signal translation system.
While varactor diodes exhibit capacity changes for DC voltage changes of very small magnitude, tuning voltages below 1 volt DC are considered unusable in television tuners, because the signal levels in the tuner are often large enough to produce noticeable changes in such low tuning voltage with consequent signal distortions. At the other extreme, the internal resistance of the varactor diode increases for higher rated breakdown voltages. Thus, while greater C-V range may be attained by using diodes of higher voltage breakdown and increasing the tuning voltage, the diode internal resistance limits the gain achievable in the tuner at the high frequency channels, and is, therefore, self-defeating. This factor effectively imposes an upper limit on the tuning voltage of about 25 volts. Therefore, the usable tuning voltage range for the varactor diode is between 1 and 25 volts.
The gain-bandwidth factor of the tuner is important since it is desired to amplify signals in all portions of the television spectrum uniformly. Amplifying devices exhibit decreasing gain with increasing frequency and the high frequency gain of the tuner must, therefore, be maximized because it effectively establishes the overall tuner gain. This portion of the tuning range corresponds to minimum tuning capacitance of the varactor diode and maximum tuning voltage, since the diode operates in the depletion mode. To maintain high frequency gain, the smallest minimum value of tuning capacitance is used. On the other hand, other factors place restrictions on the value of the minimum capacitance, primarily the stray capacitance of the tuner.
During production of a tuner, alignment is performed by applying an appropriate signal (as well as numerous marker frequencies) to the tuner and observing its response characteristic on an oscilloscope. For VHF, the oscillator coil is adjusted at the high end of both the Hi and Lo bands, that is, at channels 13 and 6. Adjustment takes the form of physically distorting turns of the coil with a thin flat object and is referred to as "knifing". The physical distortion, of course, affects the inductance of the coil. Then the middle of the Hi band (channel 10) is adjusted by knifing the coils in the interstage and RF stages, followed by a similar adjustment in the middle of the Lo band (channel 4). The process may be repeated a number of times because of the tendency of the coils to "relax" after knifing and change their values.
All physical distortion or movement of the tuner components has an effect on the miscellaneous capacitance of the tuner. Other significant sources of capacitance are the tuner wiring, and the bandswitching diodes generally incorporated in tuners, although not illustrated in this invention. By far, the largest factor in tuner capacitance is contributed by the input capacitance of the signal translation means in the tuner, mainly the RF amplifier. The input capacitance of a FET amplifier is relatively fixed from device to device and is on the order of 5 pf. Preliminary investigations indicate so-called D MOS FET's may have an input capacitance of less than 5 pf. In any case, these capacities are lumped into one quantity and referred to as the stray capacitance Cs of the tuner. The tuner stray capacity thus consists of the input capacitance of the signal translation means plus the miscellaneous capacitances mentioned above and may vary somewhat from tuner to tuner.
All of these factors have dictated the use of large Cmax/Cmin varactor diodes, with at least one bandswitching in the VHF spectrum. Consequently, only varactor diodes of the hyper abrupt junction type have been used in television tuners.
As will be explained fully when analyzing the behavior patterns of the various types of varactor diodes, the gamma factor (.gamma.) for a hyper abrupt junction diode is not a constant and results in the diode C-V characteristic being unpredictable. This poses a very practical problem in tuner design where one varactor diode is used in the RF stage, another in the oscillator stage, and two others in the tuned interstage coupling network. The four diodes must "track" with tuning voltage. That is, not only the Cmax/Cmin ratio of each diode must be substantially the same, but the actual capacity exhibited at a given tuning voltage must be within very close tolerances.
The difficulties of manufacturing hyper abrupt junction varactor diodes are numerous, but among the most important are that the capacitances exhibited by the diodes at any particular tuning voltage differ, the Cmax/Cmin ratios differ and, worst of all, because the gamma factor is not a constant, neither characteristic may be reliably predicted at any other tuning voltage based on its value at any given tuning voltage. Thus stringent specifications and limits are required for television tuner varactor diodes.
In order to maintain an acceptable manufacturing yield of these diodes, elaborate diode characteristic comparison steps or "sorts" are performed to group sets of four matched diodes for a tuner. A generally accepted current industry standard specifies that the diodes in a tuner must have a capacitance value well within 1.5% of each other at four selected tuning voltages (1, 10, 15 and 25 volts DC).
The diodes are typically initially sorted into ten groups of .+-. 1.5% limit within a range of .+-. 15% of the bogey or nominal capacitance value at tuning voltages of 1 volt and 25 volts. The sorted diodes are further sorted into similar .+-. 1.5% categories at intermediate tuning voltages of 10 and 15 volts. The result is 10.sup.4 (10,000) categories to achieve groups of four diodes matched within .+-. 1.5% of each other and within .+-. 15% of the nominal value at the four selected tuning voltages.
The diodes are supplied to the tuner manufacturer in matched sets and are effectively "married" to a particular tuner. Should one of the diodes in a tuner fail or need to be replaced for any reason, all four diodes must be replaced. This not only adds to the cost of tuner manufacture but also imposes limitations on servicing, parts inventories, alignment procedures, etc.
The tuner of the invention covers the VHF frequency spectrum in two bands, uses substantially equal ranges of tuning voltages, and incorporates abrupt junction varactor diodes exhibiting highly predictable C-V characteristics. The value of minimum tuning capacitance is selected to maintain an acceptable gain over the operating bandwidth of the tuner and the tuning voltage range is optimized within the limits established by the signal handling capability of the tuner and diode breakdown voltage.
The benefits that flow from the invention are many and varied. The costly and time consuming diode sorting operations required with hyper abrupt junction diodes is eliminated. With abrupt junction diodes, these matched diodes are replaced by any randomly selected diodes of the same initial capacity.
With predictable tuning curves for the tuner (because of the predictable C-V characteristic of the diodes), the ratios between various channel tuning voltages become precise. A voltage divider network of precise resistance ratio is thus usable to establish these tuning voltages corresponding to the desired channels in the VHF television spectrum.
As will be described in conjunction with the preferred embodiment of this invention, but claimed in copending application Ser. No. 502,482, the divider networks may conveniently be printed on an alumina substrate which forms a part of a switch assembly. The resistors may be trimmed to precision ratios automatically with well-known automated laser trimming apparatus and techniques.
As will be further described, a pair of settable potentiometers are also incorporated on the substrate for establishing the end terminals of the voltage divider network at the correct tuning voltages corresponding to the limit channels in the band. These potentiometers form the subject matter of another copending application Ser. No. 494,113.
Setup of the receiver tuner in the factory is materially simplified by merely adjusting the tuning voltages at the end points of a voltage divider network to correspond to the limit television signals in the band. Correct tuning voltages for intermediate television signals are automatically obtained because of the precise ratio between resistors in the voltage divider network and the predictable C-V diode characteristic.
As will be apparent to those skilled in the art, it is also unnecessary to provide any means for "fine tuning" the television tuner. The term fine tuning is actually a misnomer as evidenced by the fact that manual fine tuning controls exist in television receivers equipped with "automatic fine tuning" systems. Fine tuning more correctly applies to the means provided to "set" the tuner oscillator to receive a particular frequency signal at the appropriate location. These means are provided because it has heretofore not been possible to make tuners of sufficient consistency to preclude their need. In accordance with another aspect of the invention, no fine tuning is needed provided the resistor network is temperature and age stabilized and coupled to a source of stable DC voltage. These criteria are clearly meetable by state of the art technology and the former attainable in a functionally and economically attractive manner by the alumina substrate switch assembly of the above-mentioned copending application Ser. No. 502,482.